The present invention is directed to a non-solidifying paste consisting of an equilibrium mixture of a liquid metal and particulate solid constituents. The present paste is thermally and electrically conductive and can be used for cooling electronics and/or making electrical connections.
The paste has many advantages over liquid metals or solders for use as a thermal interface material. As compared to liquid metals, the present paste is more viscous than liquid metals and provides containment to impulsive loading. In many cases it has higher thermal and electrical conductivity and eliminates gross dewetting from certain materials, as may be seen with liquid metals. As compared with solids, the paste is perfectly compliant in the plane parallel to the joined surfaces. This results in the elimination of thermal stress on the joined surfaces, unlike most bonds made with solid materials. Further, the paste is compliant in the direction perpendicular to the interface for height variablility tolerant assembly; however, no temperature is required to cure or harden the material, and more importantly the connection is detachable.
The cooling of electronic components in ground based and avionics systems is becoming more challenging. As the desire for greater processing speed and compactness increases, cooling systems for these applications must have an efficient, low stress, thermal interconnect between the power dissipating element and the final heat sink in the package. This thermal interconnection is the limiting factor in many cooling systems in which heat flows through the back of a chip to a heat exchanger above the substrate. Being a common problem, many techniques have been used. Most can be classified as one of either a solid metallurgical bond, a microstructural or structural contact, a demountable liquid or grease interface, or a combination of these types. Examples of materials used for a solid metallurgical bond are solders, Au-Si eutectic bonds or silver loaded epoxies. A typical thermal resistance across an interface formed using these materials is approximately 0.2 K-cm.sup.2 /W depending on the void fraction of the joint and the exact material used. While this thermal performance is acceptable for many applications, this technique is structurally unappealing except for low cost packages with single solid state devices which may be thrown away if a device fails. For multi-chip modules such a solution is almost impossible to engineer and technically unreliable unless special provisions are included in the design to allow rework and repair of the module and to account for the unavoidable positional tolerances between the backside surfaces of the different chips to be cooled. In addition, these materials require the bond to be formed at an elevated temperature which even further complicates the assembly.
An example of a microstructure interface intended to provide a low impedance thermal interface between solid state device and a cold plate or the like is given in U.S. Pat. No. 4,498,530. This example is similar to many other examples which use a microstructure to increase the area of heat transfer across the interface in an attempt to lower its thermal impedance while providing compliance. While these solutions can achieve the vertical compliance needed from a manufacturing standpoint and the compliance parallel to the surface of the device, their thermal performance is questionable since the complicated structure usually has a long thermal path which offsets the advantage of increasing the apparent area of the interface.
Greases and other grease-like materials have been used for making compliant thermal interconnects. Two examples are Dow Corning Heat Sink Compound DC-340 and the material described in U.S. Pat. No. 4,299,715. These materials have the desirable characteristic of being fluid enough to fill microscopic voids that are present when two macroscopically smooth surfaces are brought into contact. A major object of such materials is to be able to improve the heat conduction across such a joint by filling these voids. Despite the low thermal conductivity of greases (e.g. 0.01 W/cm K) in comparison to metals, these solutions are attractive because the void area at such an interface is usually greater than 90% of the apparent contact area. A disadvantage of these materials is that they are difficult to handle in production. Further, when these grease-like materials are used with packages requiring long term reliability, difficulty often arises with containing the materials within the interface and maintaining the materials in place during operation.
Liquid metal interfaces retain the desirable characteristics of a thermal grease interface and have a much lower thermal impedance. The thermal conductivity of gallium, for instance, is approximately 100 times greater than that of a thermal grease. Thus, thermal impedances below 0.05 K-cm.sup.2 /W are achievable. None of the other techniques or materials can approach impedances this low. This is the major attraction of liquid metal based interfaces. However, because liquid metals are electrically conductive, handling and containment within the interface between the hot chip and the cooling structure is even more critical than with thermal greases. Liquid metals tend to dissolve and/or amalgamate with many of the common metals used to fabricate printed wiring boards and integrated circuits. Thus, incorrectly used, the liquid metal may actually destroy the devices it is being used to cool. Moreover, initially wetting as well as maintaining wetting of the surfaces to be joined is critical to the thermal performance and containment of a liquid. Another advantage of liquid metal based materials is their ability to wet ceramics such as alumina and aluminum nitride.
Similarly, various methods and materials have been used to make electrical connections between first and second electrical components. Again, the prior art connections did not provide all of the advantages of not only low electrical resistance, but also a conformable, compliant and removable connection which is secure enough to withstand the environmental conditions to which the components are subjected.